In recent years, it has been required to improve fuel economy of an automobile for the purpose of global environment conservation, and needs for a high-strength steel sheet have been increasing in order to reduce weight of a vehicle body and to secure safety of a passenger. It is insufficient that a steel sheet used for a member for automobile has only high strength, and the steel sheet is required to have high corrosion resistance, good press formability, and good bendability.
As a hot-dip galvanized steel sheet having good elongation, a steel sheet utilizing TRIP (Transformation Induced Plasticity) effect of retained austenite is known. For example, Patent Literature 1 discloses a high-tensile hot-dip galvanized steel sheet made for the purpose of improving strength and ductility. However, if hard martensite is contained in a steel sheet for the purpose of high-strengthening, formability of the steel sheet deteriorates.
Other than the Patent Literature 1, Patent Literatures 2 to 14 disclose techniques for the purpose of improving mechanical properties of a steel sheet such as performing tempering of martensite. However, even with these conventional techniques, it is difficult to improve the elongation property and the formability of a plated steel sheet while obtaining high strength. Specifically, although the formability may be improved by performing the tempering, it is not possible to avoid reduction in strength caused by the tempering.